This invention relates generally to a laser marking system and more particularly to fiber laser marking systems operated cw or pulsed for marking surfaces of objects with information or data, hereinafter referred to as xe2x80x9cindiciaxe2x80x9d which includes, for example, alphanumeric information, letters, words, personal or company logos, tradenames, trademarks, data or batch codes, numbers, symbols, patterns, article coding or identification, personalized signatures, and the like.
Laser marking systems have been in existence as early as 1971 for marking indicia on surfaces of articles. A major use of laser marking of articles is for marking an article or product or a product package particularly with respect to high volume manufacturing lines, so as to take advantage of marking these goods xe2x80x9con-the-flyxe2x80x9d. This type of marking provides data about the product, such as, date of manufacture, shelf life, factory origin, model and/or serial number, product tracking and the like. The use of lasers to provide marking indicia is preferred since it does not generally affect the integrity of the article or product or its packaging and the marked indicia is not easily removable.
An example of traditional laser marking systems are cw or pulsed CO2 lasers and yttrium aluminum garnet (YAG), e.g., Nd:YAG lasers where the marking is accomplished by the heat of the applied laser beam. The wavelengths of the pulses produced by these systems are within the visible or infrared spectrum. A pattern or indicia to be marked is formed by using a mask through which the laser beam passes or by a focused laser beam which is moved or scanned to produce the desired indicia or pattern. Such lasers are also employed for engraving, soldering and welding wherein, the case of marking, the surface layer of the material is melted, ablated or vaporized to produce discernible indicia or pattern. Also, this type of article marking may be accomplished by use of a chemical reaction at the article surface to be marked where certain coating agents on the surface of the article, which may be visibly transparent, but undergo a visible contrast change under the influence of a laser beam or laser pulses.
CO2 lasers have been principally employed for marking plastic surfaces, such as IC packages. The laser beam from the laser is directed through a copper stencil to form the indicia on the plastic surface. However, due to the shrinkage of IC packages over the years, CO2 lasers, in many cases, are no longer suitable since high quality indicia with good visibility is no longer satisfactory for this particular application. However, low cost, lower marking quality CO2 systems employing low cost X-Y galvanometer devices are still employed for applications not requiring high quality marking.
YAG lasers are extensively employed today for IC package marking as well as many other marking applications. YAG lasers have shorter wavelengths of operation permitting the marking of indicia on harder surfaces, such as ceramic material. The beam in the YAG marking systems is steered or scanned in one, two or three dimensions by means of a pair of displaceable mirrors mounted for rotation to displace a laser beam in orthogonal directions to form a two-dimensional scan of the beam on the surface to be marked, such as, for example, a X-Y galvanometer device or a X-X galvanometer device operated under computer control. Examples of two-dimensional scanners are disclosed in U.S. Pat. Nos. 5,225,923; 5,329,090; 5,719,372; and 5,724,412. Indicia is scribed onto the surface of an article to be marked with fine resolution and marking clarity on comparatively smaller surfaces, such as in the case of smaller IC packages. A specific example of a YAG laser system for this type of marking is the scanning Nd:YAG laser called the Laser Marker SL475E, manufactured by NEC Corporation of Japan. The marking parameters of this system are as follows: (1) Laser Oscillator: SL114K, (2) Laser Type: cw Nd:YAG laser, (3) Output: 50 W or above, (4) Number of Marked Characters: 40, (5) Marking Method: One stroke or vector, (6) Power at Marked Surface: 1 W, (7) Scanning Speed: 100 mm/sec., (8) Bite Size: 30 xcexcm; and (9) Q-Switch Frequency: 3 kHz.
The disadvantage of these CO2 and YAG laser marking systems is the need in most instances for separate, expensive refrigerated chillers or water cooling units and corresponding cooler controller and power supply to maintain cooling of the cw operated laser diode arrays for pumping the YAG rod or cw operated CO2 marking lasers. The chillers are required in CO2 marking lasers due to the low efficiency in converting lamp pump light into a cw laser output.
Further, the modulation of these marking lasers is generally accomplished by means of modulating their optical output beams, such as with an acusto-optic modulator, to produce appropriate pulses for forming marking strokes or vectors that, together, form intelligent indicia on the article surface. As a result, as much as 20% to 30% of the power in the modulated output is lost due to this type of external modulation. The cw operation of these types of lasers is a waste of energy, requires continual maintenance of the lasers, and reduces their overall lifetime utility. In the pulse mode, there is a large pulse-to-pulse variation in YAG marking lasers, as they lack uniformity in the energy applied to the marking surface. Moreover, the external modulator, beside its high loss, does not last long in the field and needs to be replaced, and is an added and continuing cost to the laser marking system, along with its RF driver. Further, the YAG laser systems used for marking require first pulse suppression, i.e., when the laser is turned off the light has to be xe2x80x9cbled offxe2x80x9d. Also, these systems with their associated cooling units and large power supplies and large laser head takes up a consider amount of floor space in a manufacturing facility.
What is needed is a less expensive marking laser system that provides marking xe2x80x9cpower-on-demandxe2x80x9d, i.e., is not continually required to be continuously pumped for accomplishing the marking process, and taking up minimal floor space.
It is a principal object of this invention to provide a fiber laser system that provides indicia marking power-on-demand.
It is an object of this invention to provide a laser-pumped fiber laser marking system that is more compact and smaller in size than previous laser systems for marking surfaces to produce visible indicia on the surface.
It is another object of this invention to provide a laser marking system requiring no first pulse suppression.
It is another object of this invention to provide a first high power laser marking system employing a double clad fiber as the marking laser wherein its optical power output is modulated to form the marking indicia by modulation or switching ON and OFF of its pump laser, e.g., a semiconductor laser diode source.
It is a further object of this invention to provide a laser marking system that achieves high powers for surface marking accomplished with shallow surface depth significantly less than about 27 xcexcm.
According to this invention, a laser marking system comprises a high power fiber laser consisting of a double clad fiber having a doped core surrounded by an inner pump cladding and providing an optical output for marking; a high power laser diode source for pumping the double clad fiber laser via an input into the inner pump cladding; an optical scanner coupled to receive the marking output from the double clad fiber laser to scan the output over a surface of an article to be marked by sweeping the marking output in one, two or three dimensions to form strokes or vectors, the completion of which comprises indicia to be marked the article surface; and a controller to control the operation of the scanner synchronized with the modulation of the laser diode pump source to initiate the marking output and sweep and modulate the marking optical output in one, two or three dimensions to form strokes comprising the indicia. The system is capable of useful modulation rates from about 20 kHz for high contrast marking on a variety of different materials with peak pulse outputs, for example, of around 1 kW to about 5 kW, and up to cw operation especially adapted for marking thin, surface-mount packages. A main advantage of the fiber laser marking system over diode pumped and flash lamp pumped YAG laser marking systems is the ability to provide modulation via the semiconductor laser diode at the input to the marking laser rather than having to modulate the optical power beam at the output of the marking laser, such as through an acusto-optic modulator, which does not provide for a uniform pulse in terms of optical power across the pulse width and substantially decreases the amount of power in the modulated beam output. Also disclosed is circuitry to dampen or decrease the ON-time rise of a current signal input for operation of the laser diode. pump source to improve the ON-time quality of the marking optical output created by the double clad fiber marking laser.
It is believed at this writing that the fiber marking laser system disclosed herein is the only continuous single mode - fiber marking laser system commercially available in the market place.
In the laser marking system of this invention, the optical power provided for marking is power-on-demand, i.e., the diode pump lasers for the fiber marking laser are turn on and off as power is needed for forming marking strokes and vectors by means of sweeping the marking output during its ON-time period. Since the laser diode pump is not operating cw, the lifetime of the pump source is extended comparatively over the same period of time of usage.
Another advantage of the fiber laser marking system of this invention is that the modulated laser diode source can be modulated in a random ON-time period fashion with pulses produced from the fiber marking laser of any length, of any selected height at any time, i.e., when necessary on demand. In Q-switched YAG lasers, the pulsed output is limited by the Q-switch in that it can operate only with period pulse of approximately of the same pulse width.
A further advantage of the fiber laser of this invention over diode pumped YAG marking systems is that the laser diode pumping of the fiber laser occurs along the length of the fiber such as, for example, anywhere between about 20 meters to about 50 meters long, which is a length greater than one hundred times that of a YAG rod, which, may be, for example, 1 cm. to 3 cm. long. As a result, the heat generated in a laser diode pumped YAG is much greater and can readily cause thermal lensing in the YAG rod resulting in distortion of the YAG single mode operation either by laterally shifting the mode in the YAG rod or by operating in other modes other than the fundamental mode. This shifting and multiple mode operation is caused by changes in refractive index of the YAG rod because of nonuniformity of heat developed along the length of the rod due to differences in the optical power along the rod length. This phenomena can result in pulse-to-pulse variations in pulse power and in mode quality if the pulse width length or the pulse height is varied from pulse to pulse in operation of the YAG marking laser. In the use of a single mode fiber marking laser of this invention, no mode distortion occurs since the mode core waveguide of the fiber, which is in the range, for example, of 3 xcexcm to 6 xcexcm in diameter, continually maintains single mode operation.
Another feature of the fiber marking laser system of this invention is its reliability to maintain the intensity level of optical marking output by means of its feedback for controlling the current level in operation of the laser diode pump source. Thus, if there is a detection in the intensity of the marking output, the feedback control provides for increase of the current to the driver circuit for the pump laser source to increase the marking output of the fiber marking laser to its original intensity level.
A further feature of the fiber marking laser system of this invention is a continuous single mode marking output that is flexible in that the output is directly coupled to the optical input of a scanner without need of initial alignment or subsequent realignment in the field as is true in the case of YAG laser marking systems wherein periodic realignment of the YAG marking laser optical cavity and output relative to scanner input is necessary.
The efficiency and effectiveness of the fiber laser marking system over YAG marking systems is substantial in that (1) the number of laser diode pump lasers is significantly less in number enhancing system reliability, (2) the optical conversion efficiency is significantly higher, (3) the power of operation is much lower, such in the range of only 2 volts to 5 volts DC, (4) no consummables are required for replacement such as laser diode pump sources or flash lamps which burn out comparatively faster in YAG systems due to their cw operation, and (5) improved reliability of the particular laser diodes used as pump sources due to their higher wavelength of operation, e.g., 915 nm for the pump sources used in the systems of this invention versus 810 nm employed in YAG pump systems.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.